Fan J, Wang J, Ye C
Department of Chemistry, Henan Normal University, Xinxiang, Henan, 453002, China.
Talanta. 1998 Aug;46(6):1285-92. doi: 10.1016/s0039-9140(97)00395-0.
The acid dissociation constants (K(a)), base dissociation constants (K(b)) and the autoprotolysis constants (K(s)) for 2,2'-bipyridyl in water and in water+alcohol(methanol, ethanol, iso-propanol) mixed solvents have been determined at 25 degrees C and an ionic strength of 0.1 mol l(-1), from a direct potentiometric method based on the treatment of the data of a single pH titration. It has been shown that K(a) increases, whereas K(b) and K(s) decrease, with increasing proportion of the alcohol in the mixed solvents. Linear relations between pK(a), pK(b), pK(s) and the mole fraction of the alcohol were observed in the composition range investigated. These results are discussed in terms of the properties of solvent and the interactions of the different species existing in dissociation equilibrium with solvents. It is concluded that the higher stabilization of both 2,2'-bipyridyl and its protonated form by dispersion forces and of the proton by its interaction with solvent molecules in the mixed solvents compared with that in water are largely responsible for the observed changes of pK(a) with composition. On the other hand, the low stabilization of OH(-) in the mixed solvents relative to that in water and the electrostatic effect are the main factors in determining the solvent effect on pK(b).
通过基于单次pH滴定数据处理的直接电位法，在25℃和离子强度为0.1 mol l⁻¹的条件下，测定了2,2'-联吡啶在水以及水+醇（甲醇、乙醇、异丙醇）混合溶剂中的酸解离常数（K(a)）、碱解离常数（K(b)）和自质子解离常数（K(s)）。结果表明，随着混合溶剂中醇比例的增加，K(a)增大，而K(b)和K(s)减小。在所研究的组成范围内，观察到pK(a)、pK(b)、pK(s)与醇的摩尔分数之间存在线性关系。根据溶剂性质以及解离平衡中存在的不同物种与溶剂之间的相互作用对这些结果进行了讨论。得出的结论是，与水中相比，混合溶剂中2,2'-联吡啶及其质子化形式通过色散力实现的更高稳定性以及质子通过与溶剂分子相互作用实现的更高稳定性，在很大程度上导致了观察到的pK(a)随组成的变化。另一方面，混合溶剂中OH⁻相对于水中的低稳定性以及静电效应是决定溶剂对pK(b)影响的主要因素。
